libguestfs/lib/errors.c

/* libguestfs
 * Copyright (C) 2009-2020 Red Hat Inc.
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 */

/**
 * This file handles errors, and also debug, trace and warning messages.
 *
 * Errors in libguestfs API calls are handled by setting an error
 * message and optional errno in the handle.  The caller has the
 * choice of testing API calls to find out if they failed and then
 * querying the last error from the handle, and/or getting a callback.
 *
 * From the point of view of the library source, generally you should
 * use the C<error> or C<perrorf> macros along error paths, eg:
 *
 *  if (something_bad) {
 *    error (g, "something bad happened");
 *    return -1;
 *  }
 *
 * Make sure to call the C<error> or C<perrorf> macro exactly once
 * along each error path, since the handle can only store a single
 * error and the previous error will be overwritten.
 */

#include <config.h>

#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>
#include <string.h>
#include <errno.h>
#include <libintl.h>

#include "c-ctype.h"

#include "guestfs.h"
#include "guestfs-internal.h"

/* How errors and error handlers works in the handle:
 *
 * The handle has a g->error_data field which is a thread-local
 * storage (TLS) key.
 *
 * We use TLS because we want to support the common idioms of:
 *   if (guestfs_foo (g) == -1)
 *     printf ("%s\n", guestfs_last_error (g));
 * and:
 *   guestfs_push_error_handler (g, ...);
 *   guestfs_foo (g);
 *   guestfs_pop_error_handler (g);
 * neither of which would ordinarily be safe when using the same
 * handle from multiple threads.
 *
 * In each thread, the TLS data is either NULL or contains a pointer
 * to a 'struct error_data'.
 *
 * When it is NULL, it means the stack is empty (in that thread) and
 * the default handler (default_error_cb) is installed.
 *
 * As soon as the current thread calls guestfs_set_error_handler,
 * guestfs_push_error_handler, or an error is set in the handle (calls
 * like guestfs_int_perrorf and so on), the key is created and
 * initialized with a pointer to a real 'struct error_data'.
 *
 * All the 'struct error_data' structures associated with one handle
 * are linked together in a linked list, so that we are able to free
 * them when the handle is closed.  (The pthread_key* API doesn't give
 * us any other way to do this, in particular pthread_key_delete
 * doesn't call the destructor associated with the key).
 */

static void default_error_cb (guestfs_h *g, void *data, const char *msg);

/* Stack of old error handlers. */
struct error_cb_stack {
  struct error_cb_stack   *next;
  guestfs_error_handler_cb error_cb;
  void *                   error_cb_data;
};

/* Error data, stored in thread-local storage in g->error_data key. */
struct error_data {
  /* Linked list of error_data structs allocated for this handle. */
  struct error_data *next;

  char *last_error;             /* Last error on handle. */
  int last_errnum;              /* errno, or 0 if there was no errno */

  /* Error handler and stack of old error handlers. */
  guestfs_error_handler_cb   error_cb;
  void *                     error_cb_data;
  struct error_cb_stack     *error_cb_stack;
};

static void
free_error_data (struct error_data *error_data)
{
  struct error_cb_stack *p, *next_p;

  free (error_data->last_error);
  for (p = error_data->error_cb_stack; p != NULL; p = next_p) {
    next_p = p->next;
    free (p);
  }
  free (error_data);
}

/* Free all the error_data structs created for a particular handle. */
void
guestfs_int_free_error_data_list (guestfs_h *g)
{
  struct error_data *p, *next_p;

  ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&g->error_data_list_lock);

  for (p = g->error_data_list; p != NULL; p = next_p) {
    next_p = p->next;
    free_error_data (p);
  }

  g->error_data_list = NULL;
}

/* Get thread-specific error_data struct.  Create it if necessary. */
static struct error_data *
get_error_data (guestfs_h *g)
{
  struct error_data *ret;

  ret = pthread_getspecific (g->error_data);

  /* Not allocated yet for this thread, so allocate one. */
  if (ret == NULL) {
    ret = safe_malloc (g, sizeof *ret);
    ret->last_error = NULL;
    ret->last_errnum = 0;
    ret->error_cb = default_error_cb;
    ret->error_cb_data = NULL;
    ret->error_cb_stack = NULL;

    /* Add it to the linked list of struct error_data that are
     * associated with this handle, so we can free them when the
     * handle is closed.
     */
    {
      ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&g->error_data_list_lock);
      ret->next = g->error_data_list;
      g->error_data_list = ret;
    }

    /* Set the TLS to point to the struct.  This is safe because we
     * should have acquired the handle lock.
     */
    pthread_setspecific (g->error_data, ret);
  }

  return ret;
}

const char *
guestfs_last_error (guestfs_h *g)
{
  ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&g->lock);
  return get_error_data (g)->last_error;
}

int
guestfs_last_errno (guestfs_h *g)
{
  ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&g->lock);
  return get_error_data (g)->last_errnum;
}

static void
set_last_error (guestfs_h *g, int errnum, const char *msg)
{
  struct error_data *error_data = get_error_data (g);

  free (error_data->last_error);
  error_data->last_error = strdup (msg);
  error_data->last_errnum = errnum;
}

/**
 * Print a warning.
 *
 * Code should I<not> call this function directly.  Use the
 * S<C<warning (g, fs, ...)>> macro.
 *
 * Warnings are printed unconditionally.  We try to make these rare:
 * Generally speaking, a warning should either be an error, or if it's
 * not important for end users then it should be a debug message.
 */
void
guestfs_int_warning (guestfs_h *g, const char *fs, ...)
{
  va_list args;
  CLEANUP_FREE char *msg = NULL;
  int len;

  va_start (args, fs);
  len = vasprintf (&msg, fs, args);
  va_end (args);

  if (len < 0) return;

  guestfs_int_call_callbacks_message (g, GUESTFS_EVENT_WARNING, msg, len);
}

/**
 * Print a debug message.
 *
 * Code should I<not> call this function directly.  To add debug
 * messages in the library, use the S<C<debug (g, fs, ...)>> macro.
 * The macro checks if C<g-E<gt>verbose> is false and avoids the
 * function call, meaning the macro is more efficient.
 */
void
guestfs_int_debug (guestfs_h *g, const char *fs, ...)
{
  va_list args;
  CLEANUP_FREE char *msg = NULL;
  int len;

  /* The cpp macro "debug" has already checked that g->verbose is true
   * before calling this function, but we check it again just in case
   * anyone calls this function directly.
   */
  if (!g->verbose)
    return;

  va_start (args, fs);
  len = vasprintf (&msg, fs, args);
  va_end (args);

  if (len < 0) return;

  guestfs_int_call_callbacks_message (g, GUESTFS_EVENT_LIBRARY, msg, len);
}

/**
 * Print a trace message.
 *
 * Do not call this function.  All calls are generated automatically.
 */
void
guestfs_int_trace (guestfs_h *g, const char *fs, ...)
{
  va_list args;
  CLEANUP_FREE char *msg = NULL;
  int len;

  va_start (args, fs);
  len = vasprintf (&msg, fs, args);
  va_end (args);

  if (len < 0) return;

  guestfs_int_call_callbacks_message (g, GUESTFS_EVENT_TRACE, msg, len);
}

/**
 * Set the last error and errno in the handle, and optionally raise
 * the error callback if one is defined.
 *
 * If you don't need to set errno, use the S<C<error (g, fs, ...)>>
 * macro instead of calling this directly.  If you need to set errno
 * then there is no macro wrapper, so calling this function directly
 * is fine.
 */
void
guestfs_int_error_errno (guestfs_h *g, int errnum, const char *fs, ...)
{
  va_list args;
  CLEANUP_FREE char *msg = NULL;
  int err;
  struct error_data *error_data = get_error_data (g);

  va_start (args, fs);
  err = vasprintf (&msg, fs, args);
  va_end (args);

  if (err < 0) return;

  /* set_last_error first so that the callback can access the error
   * message and errno through the handle if it wishes.
   */
  set_last_error (g, errnum, msg);
  if (error_data->error_cb)
    error_data->error_cb (g, error_data->error_cb_data, msg);
}

/**
 * Similar to L<perror(3)>, but it sets the last error in the handle,
 * raises the error callback if one is defined, and supports format
 * strings.
 *
 * You should probably use the S<C<perrorf (g, fs, ...)>> macro
 * instead of calling this directly.
 */
void
guestfs_int_perrorf (guestfs_h *g, const char *fs, ...)
{
  va_list args;
  CLEANUP_FREE char *msg = NULL;
  const int errnum = errno;
  int err;
  char buf[256];
  struct error_data *error_data = get_error_data (g);

  va_start (args, fs);
  err = vasprintf (&msg, fs, args);
  va_end (args);

  if (err < 0) return;

  strerror_r (errnum, buf, sizeof buf);

  msg = safe_realloc (g, msg, strlen (msg) + 2 + strlen (buf) + 1);
  strcat (msg, ": ");
  strcat (msg, buf);

  /* set_last_error first so that the callback can access the error
   * message and errno through the handle if it wishes.
   */
  set_last_error (g, errnum, msg);
  if (error_data->error_cb)
    error_data->error_cb (g, error_data->error_cb_data, msg);
}

void
guestfs_set_out_of_memory_handler (guestfs_h *g, guestfs_abort_cb cb)
{
  ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&g->lock);
  g->abort_cb = cb;
}

guestfs_abort_cb
guestfs_get_out_of_memory_handler (guestfs_h *g)
{
  ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&g->lock);
  return g->abort_cb;
}

void
guestfs_set_error_handler (guestfs_h *g,
                           guestfs_error_handler_cb cb, void *data)
{
  ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&g->lock);
  struct error_data *error_data;

  error_data = get_error_data (g);
  error_data->error_cb = cb;
  error_data->error_cb_data = data;
}

guestfs_error_handler_cb
guestfs_get_error_handler (guestfs_h *g, void **data_rtn)
{
  ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&g->lock);
  struct error_data *error_data = get_error_data (g);

  if (data_rtn) *data_rtn = error_data->error_cb_data;
  return error_data->error_cb;
}

void
guestfs_push_error_handler (guestfs_h *g,
                            guestfs_error_handler_cb cb, void *data)
{
  ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&g->lock);
  struct error_data *error_data;
  struct error_cb_stack *old_stack;

  error_data = get_error_data (g);
  old_stack = error_data->error_cb_stack;
  error_data->error_cb_stack = safe_malloc (g, sizeof (struct error_cb_stack));
  error_data->error_cb_stack->next = old_stack;
  error_data->error_cb_stack->error_cb = error_data->error_cb;
  error_data->error_cb_stack->error_cb_data = error_data->error_cb_data;

  guestfs_set_error_handler (g, cb, data);
}

void
guestfs_pop_error_handler (guestfs_h *g)
{
  ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&g->lock);
  struct error_data *error_data;
  struct error_cb_stack *next_stack;

  error_data = get_error_data (g);
  if (error_data->error_cb_stack) {
    next_stack = error_data->error_cb_stack->next;
    guestfs_set_error_handler (g, error_data->error_cb_stack->error_cb,
                               error_data->error_cb_stack->error_cb_data);
    free (error_data->error_cb_stack);
    error_data->error_cb_stack = next_stack;
  }
  else {
    error_data->error_cb = default_error_cb;
    error_data->error_cb_data = NULL;
  }
}

static void
default_error_cb (guestfs_h *g, void *data, const char *msg)
{
  fprintf (stderr, _("libguestfs: error: %s\n"), msg);
}

/* When tracing, be careful how we print BufferIn parameters which
 * usually contain large amounts of binary data (RHBZ#646822).
 */
void
guestfs_int_print_BufferIn (FILE *out, const char *buf, size_t buf_size)
{
  size_t i;
  const size_t orig_size = buf_size;

  if (buf_size > 256)
    buf_size = 256;

  fputc ('"', out);

  for (i = 0; i < buf_size; ++i) {
    if (c_isprint (buf[i]))
      fputc (buf[i], out);
    else
      fprintf (out, "\\x%02x", (unsigned char) buf[i]);
  }

  fputc ('"', out);

  if (orig_size > buf_size)
    fprintf (out,
             _("<truncated, original size %zu bytes>"), orig_size);
}

void
guestfs_int_print_BufferOut (FILE *out, const char *buf, size_t buf_size)
{
  guestfs_int_print_BufferIn (out, buf, buf_size);
}

/* Some standard error messages for common failures. */

#define DEBUG_ADVICE                                                    \
  "Do:\n"                                                               \
  "  export LIBGUESTFS_DEBUG=1 LIBGUESTFS_TRACE=1\n"                    \
  "and run the command again.  For further information, read:\n"        \
  "  http://libguestfs.org/guestfs-faq.1.html#debugging-libguestfs\n"   \
  "You can also run 'libguestfs-test-tool' and post the *complete* output\n" \
  "into a bug report or message to the libguestfs mailing list."

/**
 * Raise a launch failed error in a standard format.
 *
 * Since this is the most common error seen by people who have
 * installation problems, buggy qemu, etc, and since no one reads the
 * FAQ, describe in this error message what resources are available to
 * debug launch problems.
 */
void
guestfs_int_launch_failed_error (guestfs_h *g)
{
  if (g->verbose)
    error (g, _("guestfs_launch failed, see earlier error messages"));
  else
    error (g, _(
		"guestfs_launch failed.\n"
		"This usually means the libguestfs appliance failed to start or crashed.\n"
		DEBUG_ADVICE));
}

/**
 * Raise an error if the appliance unexpectedly crashes after launch.
 */
void
guestfs_int_unexpected_close_error (guestfs_h *g)
{
  if (g->verbose)
    guestfs_int_error_errno (g, EPIPE,
                             _("appliance closed the connection unexpectedly, "
                               "see earlier error messages"));
  else
    guestfs_int_error_errno (g, EPIPE,
                             _("appliance closed the connection unexpectedly.\n"
                               "This usually means the libguestfs appliance "
                               "crashed.\n"
                               DEBUG_ADVICE));
}

/**
 * Raise an error if the appliance hangs during launch.
 */
void
guestfs_int_launch_timeout (guestfs_h *g)
{
  if (g->verbose)
    error (g, _("appliance launch timed out, see earlier error messages"));
  else
    error (g, _(
		"appliance launch timed out.\n"
		"This usually means the kernel or appliance hung during launch.\n"
		DEBUG_ADVICE));
}

/**
 * Raise an error if an external command fails.
 *
 * C<status> is the status code of the command (eg. returned from
 * L<waitpid(2)> or L<system(3)>).  This function turns the status
 * code into an explanatory string.
 */
void
guestfs_int_external_command_failed (guestfs_h *g, int status,
				     const char *cmd_name, const char *extra)
{
  const size_t len = 80 + strlen (cmd_name);
  CLEANUP_FREE char *status_string = safe_malloc (g, len);

  guestfs_int_exit_status_to_string (status, cmd_name, status_string, len);

  if (g->verbose) {
    if (!extra)
      error (g, _("%s, see debug messages above"), status_string);
    else
      error (g, _("%s: %s: %s, see debug messages above"),
             cmd_name, extra, status_string);
  }
  else {
    if (!extra)
      error (g, _("%s.\n"
		  "To see full error messages you may need to enable debugging.\n"
		  DEBUG_ADVICE),
             status_string);
    else
      error (g, _("%s: %s: %s.\n"
		  "To see full error messages you may need to enable debugging.\n"
		  DEBUG_ADVICE),
             cmd_name, extra, status_string);
  }
}